The present invention relates generally to nickel rechargeable cells, such as nickel metal hydride (NiMH) cells, and more specifically to a method and apparatus for automatically reversibly terminating a cell charging process. This invention can also be employed in nickel cadmium (NiCd) cells.
For greater convenience and portability, many modem electrical appliances and consumer products can be operated to draw electric current from batteries of standard size and electrical performance. For convenience and economy, various rechargeable batteries have been developed, such as nickel metal hydride cells and the like.
Metal hydride cell technology provides excellent high-rate performance at reasonable cost when compared to nickel cadmium and lithium ion technology. Moreover, metal hydride cells have about a 50% higher volumetric energy density than NiCd cells and about equal to lithium ion cells. The internal chemistry of metal hydride rechargeable cells has an impact on the ability to charge such cells. Issues affecting the ability to charge nickel rechargeable cells arise as a result of the internal chemistry of such cells. When a nickel rechargeable cell approaches a full charge state, oxygen is generated at the positive electrode as follows:4OH−→O2(gas)+2H2O+4e−
The oxygen gas diffuses across a gas-permeable separator to the negative electrode where it is recombined into cadmium hydroxide or water as follows:½O2(gas)+H2O+Cd→Cd(OH)2+Heat @ Cadmium negative electrode½O2(gas)+H2→H2O+Heat @ Hydride negative electrode
When recharging such cells, it is important to ascertain when the cell has become fully charged. For example, if a cell were to become overcharged for an extended period of time, the pressure buildup within the cell could cause the cell to fail as well as electrolyte to leak, thereby further subjecting the charger to potential damage.
Metal hydride rechargeable cells are typically recharged by applying a constant current rather than constant voltage to the cells. In this scheme, cell voltage increases gradually until the cell approaches full charge whereupon the cell voltage peaks. As the cells reach the overcharge state, the released heat causes the cell temperature to increase dramatically, which in turn causes the cell voltage to decrease. Cell pressure also rises dramatically during overcharge as oxygen gas is generated in quantities larger than the cell can recombine. Thus, conventional constant current charge interruption methods cannot support a very fast charge rate without risking internal pressure buildup, rupture, and electrolyte leakage. For this reason, metal hydride cells can be provided with safety vents.
With constant voltage charge, on the other hand, the charging current is high at the beginning of the charge, when the cell can accept higher currents, and then decreases to lower levels as the cell approaches full charge. When constant voltage charging, the above-noted signals for the end of a constant current charge process are not useful because as the cell approaches the fill charge state, the cell voltage becomes constant. Like a constant current charge approach, charging time cannot be used for the constant voltage charge when the charge rate is higher than 0.3C due to runaway accumulation of in-cell pressure that can damage devices. As a result of these shortcomings it has been difficult to identify an effective termination signaling means and constant voltage charging for metal hydroxide cells has therefore been generally considered to be impractical.
With alternating current charge, the charging current can be modulated at a defined frequency or combination of frequencies to produce a net positive current that enables the cell to become charged. An alternating current charge can provide a fast charge with less pressure buildup and lower temperature increase than constant current or constant voltage charge. However, when using an alternating current charge, the above-noted signals for the end of a constant current charge process are not useful because as the cell approaches the full charge state, changes in the cell voltage are difficult to detect above the voltage response to the applied alternating current. As a result it has been difficult to identify an effective termination signaling means and alternating current charging for metal hydroxide cells has also therefore been generally considered to be impractical. It should be appreciated that an alternating current charge is used throughout the present disclosure to mean a varying current that produces a net positive charge, such as a modulated alternating current. For example, an alternating current can be half-wave rectified or full-wave rectified to produce a series of current pulses, or an alternating current can be offset by a desired DC current.
One common way to reduce pressure buildup at the full-charge state is to provide a negative electrode having an excess capacity of greater by 40-50% more than the positive electrode, a gas-permeable separator, and limited electrolyte to accommodate effective diffusion of gasses. This avoids the production of hydrogen gas at the negative electrode while permitting the oxygen to recombine with the negative electrode material. When a cell reaches full charge, oxygen gas continues to be produced at the positive electrode, but hydrogen is not produced from the negative electrode. If hydrogen were produced, the cell could rupture from excess pressure. The oxygen recombination reaction therefore controls the cell pressure. The oxygen gas then crosses the separator and reacts with the negative electrode material. Downsides of this arrangement include reduced cell capacity and corresponding shorter cell cycle life due to degradation of the negative electrode from overcharge with oxidation and heat.
Charge termination based on peak voltage can be unreliable at the end of the charging period because an over-voltage condition can exist before termination. Termination based on a voltage decline (−dV) is necessarily associated with oxygen recombination and the accompanying detrimental temperature rise. In practice, this means that voltage detection must be accurate and fast. Unless the ambient temperature is steady, it can be difficult to accurately measure a change in voltage. Moreover, when the charge rate is slower than 0.3 C, the voltage drop measurement is too small to be detected accurately. A charge rate of 1 C draws a current equal to the rated capacity of the electrochemical cell or battery. Termination based only on peak temperature is also easily affected by ambient temperature changes.
Others in the art have sought pressure-based mechanisms for reversibly breaking the connection between the electrode and the cell terminal when internal cell pressure exceeds a predetermined level. Such systems have proven useful for their intended purpose, but require volume-occupying components to be installed in the cell.
Still others have attempted to determine a charging termination point based upon the rate of change in temperature over time (dT/dt). This charging method is useful because an increase in temperature slope provides an indication that cell charging should be terminated prior to an indication based on absolute temperature. Charge can thus be terminated before damaging pressure accumulates within the cell. Accordingly, there is a reduced risk of cell rupture and leakage in a dT/dt method when compared to the other methods noted above. This makes it the most common charge termination method in use today.
Conventional battery packs include thermistors that are attached to the sides of the cells disposed in the packs. A processor in the charger receives the temperature data and determines the rate of temperature change over time (dT/dt). Once dT/dt exceeds a specified threshold, the charger discontinues current to the battery pack. Because the sides of the cells in battery packs are typically conductive, the heat transfer rates from the cell interior to the thermistor are sufficient for the thermistor to provide a suitable indication of thermal behavior inside the charger. The processor is thus able to produce a reliable indication of the charge termination point based on temperature increase.
Single cells (i.e., cells not in battery packs), however, typically have insulating labels around the outer surface of the can to enable safe handling. The labels also provide decreased heat transfer rates from the interior of the cell. Thermistors therefore may not be placed at the side of individual cells if meaningful temperature data is to be obtained. Accordingly, conventional charge methods for round cells mount a thermistor directly on the charge contact that engages the negative end of the cell. Heat transfer rates between the cell interior and the conductive negative end of the cell have proven suitable for use with the thermistor. However, high charging currents also pass through the contacts that generate a significant amount of IR heat that is emitted by the charge contacts and sensed by the thermistor. As the temperature of the thermistor increases, the measurement variations in temperature slope become increasingly unreliable. The resistance of the interface between the charge contact and cell being charged coupled with the additional IR heat that is produced interferes with the thermistor's ability to accurately determine the rate of temperature change at the negative end of the cell. Accordingly, conventional chargers measuring single cell temperatures do so with low sensitivity to temperature change, and are thus unable to determine a charge termination point prior to the accumulation of potentially damaging internal cell pressure.
In summary, as a single (i.e., not in a battery pack) metal hydride rechargeable cell reaches its fully charged state, oxygen is evolved from the positive electrode, thereby increasing the internal cell pressure and driving the exothermic oxygen recombination reaction. At a very high constant current charge rate, usually less than one hour, charge current is still very high at the end of charge. This results in severe heating of the cell and shortened cycle life. The available methods of terminating constant current charge of single cells based on rate of temperature change have not proven to be reliable due to the placement of the thermistor along with the significant amount of heat generated at the negative end of the cell.
What is therefore needed is a method and apparatus for reliably determining a charge termination point relying on a measured change of temperature over time. It would be further desirable to reduce the thermal effects of charging on the electrochemical cell being charged in order to prolong the charging cycle.